<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .div1 {
            height: 100px;
            border: 1px solid #ff0000;
        }

        .absolute-div3 {
            position: absolute;
            left: 0px;
        }
    </style>
</head>
<body>
    <div class="div1">

    </div>
    <div class="div2">
        <div class="absolute-div3">
            我是div3-绝对定位
        </div>
    </div>
</body>
</html>